PhD Defense by Damien THOMAS the 15th of December 2025

Jury:  

Stéphane VINCENT, Reviewer, Université Gustave-Eiffel

Lounès TADRIST, Reviewer, Université Aix-Marseille

Christophe JOSSERAND, Member, Institut Polytechnique de Paris, LadHyX

Stéphane POPINET, Member, Sorbonne Université

Christophe JOSSET, Member, Université de Nantes

Stéphane ZALESKI, PhD Director, Sorbonne Université, CNRS, Jean Le Rond d'Alembert

Jean MOUREH, Member Université Paris Saclay, INRAE FRISE

Stéphanie LACOUR, PhD Director, Université Paris Saclay, INRAE FRISE

Abstract : 

In the refrigeration sector, improving the energy efficiency of heat exchangers is a major challenge. Among the methods studied, spraying water directly onto the exchangers appears to be an effective strategy for enhancing heat transfer through evaporative cooling. However, when applied to complex geometries such as fins, the sprayed water does not evaporate instantly and instead accumulates locally by coalescence in the form of thick films. This clogging phenomenon blocks the gaps available for air circulation, which greatly degrades overall thermal performance. This thesis focuses specifically on these obstruction mechanisms as well as the effect of wettability properties on heat transfer, with the aim of identifying solutions that improve cooling performance while optimizing water consumption.

The first part is devoted to the study of clogging. Experiments carried out with metallic grids placed in the spray cone showed that the mesh geometry plays a decisive role. Large meshes limit obstruction, whereas meshes smaller than the capillary length promote film spreading and the formation of water films that block air flow. Numerical simulations confirmed the progressive formation of interconnected films around the meshes, reproducing the clogging phenomenon.

To better understand the physical mechanisms involved, a simplified study was then conducted on a single droplet confined between two fins, representing the post-coalescence state. Three main parameters were identified: the Bond number Bo, the aspect ratio L/D between the channel width and the droplet diameter, and the contact angle. The results show that hydrophilic surfaces promote the entry of small droplets into the channels due to capillary forces but prevent their evacuation, increasing the risk of persistent obstruction. In contrast, hydrophobic surfaces prevent liquid entry but facilitate its removal. Large droplets with large Bo and small L/D deform and break up at the channel entrance, which disrupts their transport and promotes blockage. Conversely, droplets of a size comparable to that of the channel do not fragment and are more easily evacuated.

The second part deals with surface wettability and its role in cooling by liquid film under small spraying flow rates, in order to promote low water consumption. On hydrophobic surfaces, water forms isolated droplets that slide rapidly; liquid/wall thermal contact is limited, and mainly convective heat transfer remains weak. On surfaces with intermediate wettability, droplets coalesce and form a continuous film, convective exchange increases, and cooling improves with water flow rate. On hydrophilic surfaces, the film is well spread and thin, especially in lateral areas, which intensifies evaporation and improves thermal efficiency even at small flow rates. However, part of the liquid may stagnate without actively participating in heat exchange. Two heat transfer regimes were identified: a predominantly evaporative regime at very small flow rates and a mixed convection/evaporation regime when the flow rate increases. Water flow must therefore be sufficient to avoid surface drying and to promote evaporation. Measurements of film thickness, flow velocity and dissipated heat flux helped relate wettability to thermal efficiency, while providing reference data for the validation of numerical models of thin film flows.